(1) Field of the Invention
The present invention relates to a process for producing an ultrahigh silicon electrical steel sheet having an excellent magnetic property for use as a soft magnetic material in an iron core of electrical machinery and apparatuses by cold rolling, and having an excellent workability. According to the present invention, it becomes possible to produce an ultrahigh silicon electrical thin steel sheet having a small thickness best suited for use in an iron core of electrical machinery and apparatuses, particularly high-frequency electrical machinery and apparatuses.
(2) Description of the Related Art
A steel sheet containing silicon has been used as an iron core of a power transformer or rotating machine, due to its excellent soft magnetic property. In this soft magnetic material, the iron loss property improved, i.e., the iron loss value lowered, with an increase of the silicon content. In particular, when the silicon content is around 6.5%, the iron loss property is good, and further, the magnetic struction approaches zero, which contributes to a further improvement of the magnetic permeability, and thus a magnetic material having a new function not attained by the prior art can be obtained. Iron having a silicon content of 6.5%, however, has various problems in the cold working thereof, for example, cold rolling, and therefore, has not been put to practical use.
Examples of the problems encountered in the cold working of the iron having a silicon content of 6.5% include the following.
1) Due to a small elongation derived from the intrinsic property of the high silicon iron crystal, the iron is susceptible to sheet breaking during the cold rolling. PA1 2) Due to a small elongation inherent in the high silicon iron, the iron is liable to cause cracking at an edge portion of a sheet, i.e., edge cracking, during the cold rolling. PA1 3) Since the high silicon iron has a very high hardness, the rolling load during cold rolling becomes very high when the final thickness is small.
Recently, high silicon steel sheets having a silicon content of 6.5% or around 6.5% have been reconsidered as energy saving or as novel magnetic steel sheets capable of meeting various magnetic property requirements of electrical machinery and apparatuses. In particular, a great effort has been made to solve the problem of cold rolling, and this has led to various proposals. For example, in connection with the problem of the high susceptibility of the high silicon iron to sheet breaking described in the above item 1), Nakaoka et al. proposed in Japanese Unexamined Patent Publication (Kokai) No. 61-166923 a method wherein continuous finish hot rolling conditions are specified on a hot rolled sheet used as a material for cold rolling, to thus form a metallic structure extending in a fibrous form to the rolling direction. Nakaoka et al. proposed in Japanese Unexamined Patent Publication (Kokai) No. 62-103321 a method wherein a metallic structure in a fibrous form stretched in the rolling direction is formed by determining a crystal grain size of a material before a continuous finish hot rolling. In these methods, the hot rolled sheet structure is controlled by determining the continuous finish hot rolling conditions, and the cold rolling is made possible through the use of the resulting hot rolled sheet as a starting material.
Alloying of an iron having a silicon content of 6.5% with a third element has been reported as a method of improving the cold rollability. For example, C. A. Clark et al. reported in IEE., 113 (1966) p. 345, an effected attained by an addition of nickel, and K. Narita et al. has reported in IEEE Trans. Mag. MAG-14 (1978) p. 258 an effect attained by an addition of manganese.
Further, Kimura et al. disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-299702 a method and an apparatus for carrying out rolling at a temperature of 350.degree. to 450.degree. C. The conventional cold a rolling technique, however, cannot cope with the above-described temperature range.
The problem of edge cracking described in the above item 2) can be solved by a method capable of solving the problem described in the above item 1). Further, to prevent edge cracking, a more careful application of a method generally used in other types of steels is useful also for a cold rolling of a high silicon steel. For example, Masuda et al. proposed in Japanese Unexamined Patent Publication (Kokai) No. 62-295003 to prevent edge cracking through a control of a heat crown at the roll end portion.
The problem of an excessive rolling load described in the above item 3) is such that the hardness (Hv) of steel increases with an increase in the silicon content and, for example, reaches 390 when the silicon content is 6.5%, so that the cold rolling load becomes too high. The thinner the rolling thickness, the larger the rolling load. In general, when the diameter of the rolling rolls is reduced, the contact arc length between the rolls and the rolling material becomes small, which enables a sheet material to be rolled under a low load. For this reason, a Sendzimir mill provided with working rolls having a diameter of 100 mm or less has been used for the cold rolling of a grain-oriented magnetic steel sheet or non-oriented magnetic steel sheet having a silicon content of about 3%. Therefore, obviously a rolling by a rolling machine provided with working rolls having a smaller diameter is necessary for the cold rolling of a material having a silicon content of 6.5%, i.e., a material having a much higher hardness than that of the material having a silicon content of 3%, to a thin thickness. In the cold rolling of the material having a silicon content of 6.5% by a rolling machine provided with work rolls having a small diameter, however, a problem of strip breaking arises, as reported by Takada et al. in Japanese Unexamined Patent Publication (Kokai) No. 63-145716.
For this reason, the solution of the problem described in the above item 1) becomes necessary also for a rolling of the high silicon material by a rolling machine provided with working rolls having a small diameter.
The magnetic properties of a high silicon iron will now be described.
A motive for the development of a high silicon soft magnetic steel sheet resides in the realization of high functions not attained by the prior art, for example, iron loss property and magnetizing property, although the difficulty of production has fully been recognized in the art. Therefore, although it is obvious that attention should be paid to an ease of production, particularly the ease of cold rolling, it is necessary to design the manufacturing process while making the first aim the production of a product having good magnetic properties. In this respect, no satisfactory technique has been established on the process for producing a high silicon soft magnetic steel sheet, especially imparting an optimal magnetic property to a material having a silicon content of 6.5% wherein the magnetic striction becomes minimum. In particular, a reduction of the iron loss in a thin product is essential to a material exhibiting an advantage in a high frequency region, such as a steel having a silicon content of 6.5%, and the worth of this means is halved in the production of a steel having a silicon content of 6.5%, at which is impossible to produce a thin product. For example, Abe et al. avoided, in Japanese Unexamined Patent Publication (Kokai) No. 62-227035, the problem of the cold rolling by a process wherein siliconizing is conducted in an atmosphere containing SiCl.sub.4 , i.e., by the CVD process, and produced a product having a thickness of 0.10 mm; see NKK Technical Report, No. 125, 58 (1989). In the CVD process, however, problems remain unsolved with regard to the productivity and accuracy of the sheet thickness, and the development of a novel manufacturing process by the cold rolling is desired in the art. Note, Japanese Unexamined Patent Publication (Kokai) No. 62-270723 discloses a product having a thickness of 0.30 mm, and Japanese Unexamined Patent Publication (Kokai) No. 61-166923 discloses a product having a thickness of 0.50 mm. Further, also in the above-described report, which describes the effect of the component per se, the thickness of the product disclosed is as thick as 0.35 mm. This thickness is unsatisfactory for a sufficient exhibition of the advantage of the magnetic property of the steel having a silicon content of 6.5%.
It is known in the art that a material having a poor workability is rolled at an elevated rolling temperature, i.e., by a warm rolling. In the steel also having a silicon content of 6.5%, the warm rolling is less susceptible to cracking, i.e., more effective than the room temperature rolling. The warm rolling, however, has problems such as a heat resistance of rolling lubricants, a necessity to provide new equipment for ensuring the rolling temperature, and a difficulty of regulating the sheet thickness accompanying the variation in the sheet temperature in the widthwise direction and longitudinal direction of the sheet. Therefore, the warm rolling cannot be adopted as such. For example, Japanese Unexamined Patent Publication (Kokai) No. 1-299702 discloses a method and equipment for conducting rolling at a temperature of 350.degree. to 400.degree. C. In this method, a material is rolled to a thickness of 0.2 to 0.4 mm. Japanese Unexamined Patent Publication (Kokai) No. 63-36906 discloses that a material is rolled at 350.degree. C. to a thickness of 0.35 mm. In the field of the production of a grain oriented electrical steel sheet having a silicon content of about 3%, Japanese Examined Patent Publication (Kokoku) No. 54-13846 discloses that the magnetic property is improved by maintaining the material at a temperature of 50.degree. to 350.degree. C. for one min or longer, in between passes of the rolling. In an embodiment, a reverse rolling is conducted at an elevated sheet temperature. In general, the rolling at a sheet temperature of about 250.degree. C. is widely conducted for avoiding the above-described problems such as lubrication and uneven sheet temperature.